The present invention relates generally to apparatus and methods for sensing rotary or angular position, and specifically magnetic field sensors such as hall effect detectors with an arrangement particularly suited for electrical rotary actuators.
There are a variety of known magnetic sensor technologies including magnetoinductive sensors, flux-gate sensors, magnetoresitive sensors and hall effect detectors. Hall effect detectors are the smallest and least expensive of these sensors. The operating theory of a hall effect detector is simple. If the magnetic flux lines of a magnetic field impinge pependicularly on a thin panel carrying a current, a voltage develops across the sides of the panel which can be measured. Due to compact size and cost considerations, hall effect detectors have been widely used in electrical rotary actuators for industrial applications.
One traditional method of employing a hall effect detector for sensing rotary position has been to offset the hall effect detector from the axis of rotation and employ a ring magnet (e.g. having two north poles and two south poles) about the shaft. The hall effect detector and ring magnet are arranged in a plane perpendicular to the shaft axis such that when the shaft rotates, the faces of the north and south poles cyclically pass directly in front of the sensing surface of the hall effect detector. One of the significant problems with this approach occurs when the sensor is closely coupled to an electromagnetic actuator. In this application, magnetic leakage fields develop due to the wire coil and emit out the end of the device. This can interfere with the sensor signal. This leads to accuracy problems. The sensor output is also sensitive to proper sensor and ring magnet positioning (e.g. providing the proper gap between the sensor and the ring magnet).
An attempt to solve this problem has been to mount a yoke to the end of the shaft. The yoke carries two magnets on opposite sides which are adapted to rotate around the hall effect detector which is mounted to the stator stationary on the axis. The yoke thus surrounds the hall effect detector such that the hall effect detector and the magnets are arranged in a plane perpendicular to the shaft axis. When the shaft rotates, the faces of north and south poles cyclically pass directly in front of the sensing surface of the hall effect detector. Again, this method is sensitive to proper placement and the gap between the magnets and the sensor. With the hall effect detector on the axis, the magnetic leakage fields do not substantial interfere with the sensor signal. However, the implementation of this method requires extra cost and parts of the yoke/magnet assembly. This method also requires extra space at the shaft end to accommodate the yoke which is undesirable for compact applications.
It is an object of the present invention to provide a rotary position sensor and method that overcomes these and other problems existing in the art, and that may be particularly suited for electrical rotary actuators.
An apparatus for sensing rotary position, comprising an in line magnet and a hall effect detector or other suitable magnetic sensor. The magnet is axially aligned with the hall effect detector in spaced relation along an axis. The magnet has radially spaced apart north and south poles with the imaginary line between the poles intersecting the axis. With this arrangement, the hall effect detector senses relative rotation between the hall effect detector and the magnetic about the axis.
It is a significant aspect of the present invention that the novel rotary position sensing apparatus is incorporated into an electrical actuator in a novel manner. According to this aspect, the electrical actuator includes a stator comprising a lamination stack and wire coils and a rotor adapted to be rotated by the stator. The rotor comprises an output shaft carried by the stator for rotation about an axis. A magnet is fixed to an end of the output shaft and rotates in unison with the shaft. The magnet has a north pole and a south pole which emit a magnetic field having magnetic flux lines traveling in a return loop from the north to the south pole. The return loop intersects the axis. A sensor is mounted in a stationary position on the stator and axially spaced from the magnet along the axis. The sensor senses an angle of magnetic flux lines traveling along the return loop. When the shaft rotates, the magnetic flux lines rotate with the magnet to impinge upon the sensor at different angles such that the sensor has an output related to the angular position of the shaft. In the preferred embodiment, the magnet and sensor are on the center axis so that the stator field interference is minimized. Given the sensing of the return field, it is a further advantage that the sensor is less sensitive to precise positioning of the sensor in the plane normal to the axis of the shaft.
Other objectives and advantages of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.